1. Field of the Invention
The present invention relates to aqueous strippable coating compositions containing polyurethane-polyureas and polymers prepared from radically polymerizable monomers.
2. Description of the Prior Art
Aqueous coating compositions which are suitable for protecting high quality goods, in particular for the protection of lacquer surfaces of, for example, motor vehicles, are known. DE-A 19 653 585 relates to cosolvent-free, aqueous, anionic polyurethane dispersions which are also used as strippable lacquers for the temporary protection of high quality consumer goods. The tear resistance and the stretchability of the lacquer layers are relatively high.
WO 98/23692 discloses known mixtures of an aqueous copolymer dispersion of acrylic esters with an aqueous ethylene/vinyl acetate copolymer dispersion, which mixtures are used as strippable coating compositions.
The mixtures produce a satin finish protective coating, which is strippable. In a test using 36% sulfuric acid at 65° C. it was found that the clear lacquer surface provided with the protective coating is protected against initial, visually perceptible damage for at least 30 minutes longer than the unprotected clear lacquer surface.
Acrylate dispersions alone generally produce coatings which are highly weatherproof but are stiff and brittle. Soft acrylates have low tear resistance and resistance to tear propagation. The addition of thermoplastic copolymers of olefins with vinyl esters brings about a reduction in the stiffness and brittleness of the acrylate dispersions.
Thermoplastic copolymers of olefins with vinyl esters are not generally weather-resistant, however. In addition, the mixture produces a satin finish film.
Strippable coating compositions prepared from polyurethane dispersions, such as those described in DE-A 19 653 585, fulfil the requirement for transparency, weather resistance and strippability. However, the hardness, stretching and strippability of such lacquer layers need to be improved.
DE-A 19 943 933 describes mixtures of at least two mutually compatible, cosolvent-free, aqueous anionic polyurethane-polyurea dispersions A and B, which exhibit improved hardness, stretching and strippability properties. However, the cost of such high quality mixtures is high.
The users of these coating compositions desire higher hardness, lower stretchability and good tear resistance and resistance to tear propagation of the strippable coatings, and for a higher solids content of the dispersions used as strippable coating compositions. The lacquers should also be more readily strippable from a very wide variety of substrates, for example, glass, plastics and automotive coatings, than those of the prior art.
An object of the present invention is to provide aqueous polymer dispersions for strippable coating compositions which have a high solids content and may be used to prepare coatings that are transparent, weather-resistant and resistant to chemicals and, in terms of mechanical properties, fulfil all the requirements of a strippable coating composition, i.e., they can readily be stripped from the substrates and also can be produced in an economical manner.
This object may be achieved with aqueous dispersions of the present invention which are described in greater detail below.
Polyurethane-polyacrylate dispersions are well known and are preferably used as waterborne coatings for the production of base coats having good adhesion. It is therefore especially surprising, and was not predictable to persons skilled in the art, that the mixtures according to the invention and also the hybrid dispersions according to the invention (prepared by the in situ preparation of polyacrylates in polyurethanes) result in novel strippable coating compositions having excellent mechanical properties and strippability.